In recent years, a flat-bed scanner as a close-contact-type image scanning device for scanning documents and/or photos has been pervasive. The flat-bed scanner is capable of scanning a two-dimensional image by carrying out line scanning (Y direction) with a line sensor (such as a CCD line sensor) having pixels aligned in a linear pattern (X direction).
However, the scanner with such a line sensor is limited in terms of its reduction in thickness and weight due to a scanning mechanism provided for scanning a two-dimensional image, and therefore, it has some difficulties of improvement of scanning speed.
Therefore, in order to realize reduction in thickness and weight, and increase of scanning speed, an active matrix-type two-dimensional image sensor (an image scanning device) has been created. This active matrix two-dimensional image sensor is arranged so that a photo-sensing element (a photodiode, a phototransistor) and a switching element (such as a thin film transistor) are aligned in a two-dimensional manner.
This two-dimensional image sensor allows two-dimensional image scanning without using a scanning mechanism, and therefore, the thickness, the weight can be reduced to less than 1/10, and the scanning speed can be accelerated to more than 10 times, compared to the “flat bed scanner” using a conventional CCD line sensor, thus realizing an easy-to-use image scanning device.
Active matrix image scanning devices disclosed in Japanese Laid-Open Patent Application Jitsukaihei 02-8055/1990 (published on Jan. 18, 1990) and Japanese Laid-Open Patent Application Tokukaihei 05-243547/1993 (published on Sep. 21, 1993) may be examples having such a configuration.
As shown in FIG. 12, an active matrix array (an active matrix substrate) used in a conventional active matrix image scanning device is arranged so that the pixels are aligned in a XY matrix manner, and each pixel includes a thin film transistor for photo-detection (hereinafter referred to as a photo-sensor TFT), a switching thin film transistor (hereinafter referred to as a switching TFT) and a pixel capacitor (charge accumulation capacitor).
The respective photo-sensors TFT of the pixels differ to each other in charge amount accumulated in the accumulation capacitor of each pixel (or discharged from the accumulation capacitor) due to difference of the magnitude of photo-current Ip which varies according to the ratio of white to black (lightness and darkness) of the object such as a document surface. The switching TFT sequentially reads charge amount distribution (in-plane distribution) of each accumulation capacitor so as to obtain two-dimensional information of the object.
In such an active matrix image scanning device, improvement in performance of photo-sensor TFT allows scanning of the object even with a weak reference beam. More specifically, even with a low-illuminance backlight, a sufficient value can be ensured for the photo-current, thus realizing reduction of power consumption.
Further, with this arrangement, it also becomes possible to reduce time constant of charging/discharging of accumulation capacitor in the light irradiation section (not shown), and this allows high-speed scanning. Thus, some melioration has been conventionally attempted for increasing the ratio of the photo-current to the dark current, i.e., photosensitivity (Ip/Id) of a photo-sensor TFT.
For example, Japanese Laid-Open Patent Application Tokukaihei 05-243547 (Prior art 1) discloses a method for increasing the photo-current Ip upon irradiation by providing thickness difference between a gate insulation film of the photo-sensor TFT and a gate insulation film of the switching TFT, so as to improve the photosensitivity (Ip/Id). As an example of this configuration, FIG. 13 shows a configuration in which the thickness H1 of the gate insulation film of the photo-sensor TFT and the thickness H2 of the gate insulation film of the switching TFT are in a relation of H1>H2 by setting the thickness H1 to be equal to the total thickness of the first gate insulation film and the second gate insulation film, and setting the thickness H2 to be equal to the thickness of the second gate insulation film. In such a manner, this configuration provides different thickness between the gate insulation film of the photo-sensor TFT and the gate insulation film of the switching TFT.
Further, Japanese Laid-Open Patent Application Tokukaihei 02-215168 (Published on Aug. 28, 1990) and Japanese Laid-Open Patent Application Tokukaihei 06-132510 (Prior art 2: Published on May 13, 1994) disclose a configuration in which plural gate electrodes are provided in the photo-sensor TFT so as to carry out light absorption in an area less influenced by the gate voltage, on purpose to increase the ratio of the high photo-current Ip to the low dark current Id, i.e., photosensitivity (Ip/Id). For example, FIG. 14 shows another configuration for increasing the ratio of the photo-current to the dark current, i.e., photosensitivity (Ip/Id) of a photo-sensor TFT. In this arrangement, each photo-sensor TFT has two gate electrodes so as to provide on an amorphous silicon layer a light absorption area A which corresponds to a contact hole formed between the drain electrode and a source electrode.
However, though the foregoing Prior art 1 and Prior art 2 both are capable of increasing photosensitivity (Ip/Id), they still have the following problems.
Prior art 1 causes complication of manufacturing process of the TFT since the gate insulation film of the switching TFT and the gate insulation film of the switching TFT have to be made in different thickness.
Further, Prior art 2 causes difficulties of scanning for a high-definition image since provision of plural gate electrodes complicates the wiring layout for each pixel.
Therefore, the present invention provides an active matrix substrate including a photo-sensor TFT which can realize improvement of photosensitivity (Ip/Id) with a simpler structure. Further, the present invention is aimed at realizing a high-performance image scanning device capable of high-speed scanning with low power consumption (with a low-illuminance backlight).
Accordingly, the present invention is made in view of the foregoing problems, and an object is to provide a thin film phototransistor which can be used as a photo-sensor TFT capable of increasing photosensitivity (Ip/Id) without causing complication of manufacturing process or wiring layout, and to provide an active matrix substrate using the thin film transistor, and an image scanning device using the active matrix substrate.